Leming said:Is this identical to the balls hanging from strings (length d minus ball diameter) that are anchored at a common point?

Mighty nice way of transforming the problem, great thinking.It would change the difficulty setting by one or two categories I guess. I am not absolutely sure, it is difficult to visualise it , but I believe that it is unfortunately not identical.

As to the problem:I guess all the necessary information is available to solve the problem.This said, I don't see myself solving it (certainly not in a reasonable time), but I am very interested in the method used/progress made in solving it.I would first calculate the function that expresses the coordinates of the center of gravity of the 3 balls in terms of a, b, c and dThen I would evaluate this function for the center of c going from height c to height say 2c. The minimum value of this function would give you an answer where c must be.a and b coordinates can probably be calculated relatively easy, then the plane, then the angle.